In a modern axial flow turbomachine an annular stream of working fluid is conducted through one or more stages wherein energy is exchanged between the rotating turbomachine shaft and the axially flowing working fluid. In an axial flow gas turbine engine, this energy exchange takes place in both directions, with mechanical energy from the shaft transferred into the working fluid in the compressor section of the engine and the reverse occurring in the turbine section of the same engine.
As noted above, this exchange occurs in one or more stages typically comprising a rotor having a plurality of radially extending, rotating blades secured to the turbomachine shaft as well as a plurality of radially extending, fixed vanes disposed immediately upstream of the rotor. The stationary stator vanes serve to optimally direct the annular stream of working fluid into the downstream rotor blades so as to induce the desired amount of momentum transfer.
As will be appreciated by those skilled in the art, the stator vanes do not in themselves effect any transfer of energy between the turbomachine shaft and the working fluid. Rather, the stator vanes function only as a means for enabling the rotating elements of the turbomachine to more effectively interact with the working fluid. Further, it will be appreciated that an optimized velocity profile of the working fluid entering the rotor stage is desirable in order to achieve proper interaction over the spans of the individual blades.
Tests have established that the axial velocity of the working fluid in the first stage of the turbine section of a gas turbine engine is not uniform in the radial direction when measured immediately upstream of the first turbine stage rotor inlet. Specifically, the axial velocity component of the working fluid diminishes adjacent the radially inner and outer boundaries of the annular working fluid stream. Designers have attempted to redistribute the flow of working fluid exiting the stator vane stage in order to optimize the velocity profile at the vane stage exit plane and thereby improve overall engine efficiency.
One method used in the prior art to accomplish this flow distribution is the variation of the size of the nozzle throat formed between adjacent stator vanes to achieve a minimum throat dimension proximate the radial midpoint of the vane. This is accomplished in the prior art by curving the vane span in the vicinity of the vane leading or trailing edge in order to narrow the spacing between adjacent vanes at the vane span midpoint. The resulting spanwise curved vane achieves the desired mass flow redistribution at the exit of the vane stage, but its use has been accompanied by a number of operational drawbacks which have limited its effectiveness.
One drawback of the prior art curved span vane design is its tendency to induce undesirable body forces which degrade the optimum velocity profile as the flow moves axially downstream toward the rotor inlet. The resulting non-optimum velocity profile at the rotor inlet minimizes the benefits achieved from nozzle throat dimension variation.
A second drawback occurs particularly in those vanes immediately downstream of the combustor section in a gas turbine engine which require some form of internal cooling in order to withstand the high temperature environment. Curved span blades of the prior art are less easily fitted with internal cooling gas impingement structures for creating a high rate of heat transfer with a limited flow of cooling medium.
A third drawback of a curved span design vane is its non-uniform surface pressure distribution which is a direct result of the non-uniform airfoil cross-section required for nozzle throat dimension variation. The non-uniform surface pressure distribution induces a spanwise pressure gradient which in turn results in aerodynamic losses that diminish overall engine output.
What is required is a stator vane configuration which achieves and maintains the desired uniform velocity profile at the downstream rotor stage inlet while avoiding the losses and other drawbacks associated with prior art curved span vane designs.